Self-trimming mold



July 25, 1944. v. s. ANDERSON SELF-TRIMMING MOLD Filed March 50, 1942Patented July 25, 1944 UNITED STATES PATENT OFFICE SELF-TRIMMING MOLDVolney S. Anderson, Los Angeles, Calif.

Application March 30, 1942, Serial. No. 436,785

Claims.

This invention relates to plastic molds and, in some of its moreparticular aspects, it has to do with self-trimming molds and improvedmaster plates for holding mold elements.

In the molding of plastic articles, and particularly pliable articlesmolded from rubber and like compositions, it is necessary initially toplace in the mold a quantity of the composition in excess of thecapacity of the article cavity. Thus, when pressure is applied to themating mold, the excess of compound is forced out of the article cavity,between the mold plates, into a surrounding overflow cavity. After themolded article is removed from the mold, this overflow or flash portionmust be trimmed off before the article is ready for use. Although, byfollowing conventional practice, there ls usually a relatively thin webportion between the article and the fiash, at best this relatively thinweb merely defines a line along which the article must be trimmed as byshears or cutters. This heretofore necessary trimming operation hasrendered the cost relatively excessive.

It is therefore among the objects of my invention to provide what may beaptly termed a self-trimming mold-that is, a mold in which, among'itsother distinctive characteristics, the article cavity is separated fromthe overflow space or cavity by contacting shear edges which sever theoverflow flash from the article and eliminate the necessity of theobjectionable and costly trimming operation.

Another feature of my invention is a novel mold construction embodyinghighly eflicient means for providing for and controlling the flash oroverflow in a manner to produce an improved product and renderunnecessary any careful measuring of the initial charge inserted in themold. That is, in the curing of rubber compositions in the mold, gasesare released in the mold cavity, causing some gas pressure. Unlessproper control of this pressure is obtained, either the molded articleWill be objectlonably porous or blistered or else it becomes difdcult orimpossible to obtain proper closing and register or Contact of themating mold and master plates. By the novel means which I provide forcontroliably relieving the article cavity of this gas pressure and fortaking care of the flash or overilow, I am able to obtain an improvedproduct as Well as improved operation.

My invention also embraces improvements in the master plate and meansfor releasably securing the mating mold elements in properly adjustedposition.

Still further objects and corresponding advantages are inherent in myinvention and how those as well as those specifically enumeratedhereinabove are achieved will become apparent to those skilled in theart from the following detailed description of a presently preferredmold and master plate embodying my invention in one of the forms whichit may take. For this purpose I shall now refer to the accompanyingdrawing, in which:

Fig. 1 is a plan view of the master plate containing a plurality of mymold elements, parts being broken away for illustrative purposes;

Fig. 2 is a section taken on line 2-2 of Fig. 1;

Fig. 3 is a longitudinal medial section of a set of mating moldelements; and

Fig. 4 is an elevation taken on line 4-4 of Fig. 3.

In the drawing, the numeral 5 generally designates the master plate,comprising the upper half 5a and the lower half 5b; and the numeral 6generally designates a mating pair of mold plates comprising companionplates 6a and 6b.

Each of the plates 6a, 6b is shown as being round in cross-section andis rst machined to provide an article cavity 9 and adjacent overflowcavities l0, which cavities mate when the plates 6a and 6b are placed inthe master plate in face to face relation as will be hereinafterdescribed. While my invention contemplates and may employ articlecavities of any desired plan or cross-sectional shape, I here illustratethe complete cavity formed by the two mating article cavities 9 as beinground in plan and crosssection, as for the molding of a rubber ring orthe like. Each cavity 9 is separated from the adjacent overflow cavitylli by a shear edge Il preferably of the order of .005 of an inch inWidth. In longitudinal section each edge II would present microscopicundulations, in the manner of relatively sharp edges. When the matinghalves 6a, 6b are properly positioned in the cavities of the masterplate and pressure applied to the opposed halves of the master plate,edges II are the only points of the opposed faces of the halves 6a, 6bin contact. As best shown in Fig. 3, each of the overiiofw cavities Illprovided by the mating plates merges into a communicating space providedbetween the plates. These spaces are designated by the numerals l2, l2arespectively and are provided as auxiliary overiiow areas for theprincipal purpose of permitting, yet so restricting, the overflow as tocause sufficient back pressure" on the article cavity to avoidobjectionable porosity of the moldwhen. in use, that is, after theshearing edges are in conhrctr-while space I2 preferably communicateswith atmosphere.

While I have termed the overi'iow spaces I2,

|2a auxiliary overflow spaces, I do not mean to imply that they arenecessarily auxiliary to the relatively larger communicating overflowcavities Il. In fact, in some cases I may eliminate the larger ,overflowcavities III and have the spaces I2, I2a extend to the shear edges I Isurrounding the article cavity.

'I'he tolerance between the mating faces of plates la, Bb which providesthe overflow spaces l2, |2a is preferably of the order of .007 of aninch since I find this width to be adaptable for more universal use.This tolerance, however, is determined largely by the relative hardnessof the plastic composition used in the mold and the pressures employedon the mold. Typically the composition may be of a '75 Shore hardnessand the pressures employed may be typically 35-40 pounds per squareinch, and for such hardness and pressure I ilnd a tolerance of .007 ofan inch ideal. For less hardness or greater pressures, this tolerancemay be decreased slightly, or for greater hardness or lesser pressuresit may be commensurately increased.

After thus machining the mating faces of the mold plates 6a, 6b toprovide the article cavity, overilow cavities and auxiliary overflowspaces, the steel plates Ia, Bb are preferably suitably hardened beforebeing placed into use.

Fbr the purposes to be hereinafter described, I provide medially in theouter end oi each plate la, b a threaded hole Il.

With more particular reference now to the master plates a. 5b, each iscomprised of two superposed companion plates 20, 2| and parallellongitudinal spacer ribs 22, the latter being disposed along each sideedge and between each of the aligned rows of mold plate cavities 25 pro-Ivided through the inner plate 2|.

As best shown in Fig. 2. plates 20, 2l are held in superposed relationby cap screws 2l and the spacer ribs 22 are held within the longitudinalgrooves 28 provided in each inner plate 2| by cap screws 2l.

Tapered dowel pins 30, 3|, 32 are force-fitted into each of the threecavities 35, provided through plate 2| of master plate 5a, each dowelpin presenting an annular positioning flange 38; and corresponding dowelholes of uniform diameter are provided by the respective bushings 39force-fitted into the respective cavities 40 provided through the innerplate 2| of master plate lb; each of the bushings presenting an annulariiange 31 registering with the flange 38 of the adjacent dowel pin.

In each of the mold plate cavities 25 in master plate 5a, I mount one ofthe mold plates 6a and in each of said cavities in mold plate 5b I mountone of the mold plates 6b, or vice versa, so that when the master platesare superposed in the position of Fig. 2 the article cavities, overilowcavities and overflow spaces of each pair of mold plates mate, with theshear edges Il in contact with each other. In use, the shear edges aredisposed in the same plane with the mating faces asses-i1 of ribs 22,and for the purpose of eifecting this coplanar adjustment, I may provideshims Ii between the outer ends of the respective mold plates 8a, Ib andthe adjacent outer plates 2l. When thus properly positioned, I screw acap screw 41 into the threaded hole Il provided in the outer end of eachmold plate whereby to lock the mold plates in position.

In view of the substantial pressures employed on such molds and therelatively narrow shearing edges I I which are in contact with eachother. it will be readily understood that it is important to obtainproper hardening and accurate machining of the parts so that the spacerribs 22 will prevent deformation of the edges Il. Each of the masterplate elements 5a, lb is comprised of the two plates 20, 2| tofacilitate manufacture. By this procedure the mold recesses 2l may bebored entirely through the inner plate 2| and have their outer endscovered by the outer plate 20 to protect the mold elements C and toprovide means for locking said mold elements in position as described.The spacer ribs are likewise preliminarily ground on both sides andafter all said ribs are secured to the plates I surface grind themsimultaneously in their mountings to insure true coplanar surfaces.Having thus obtained true coplanar surfaces on the spacer ribs, it isrelatively simple, by employing a straight edge. to adjust the moldelements t in the recesses so that their sharp edges II will be coplanarwith the rib surfaces.

In carrying out a molding operation by the use of my device, a charge ofthe plastic composition such as rubber is placed between each of thepairs of mating heated mold elements before the master plates aresuperposed one on the other. The thus prepared mold is then placed undersuitable pressure, which forces any excess charge out of the articlecavities into the overilow cavities Il and thence into the relativelyrestricted overiiow spaces I2, I2a. After a suitable curing period, themaster plates with their carried mating mold elements are then separatedand the molded product stripped from the mold cavities in a trimmedcondition since the shearing edges will have sheared of! the iiash fromthe article.

'I'he broad concept of my invention, as deiined by the appended claims,of course may be carried out in structures varying from that hereinspecifically described, so that the details which I have hereinbeforeemployed to illustrate an adaptation of my invention are not to be takenas limitative.

I claim:

1. A device for molding plastic articles com- (/prising companion masterplates, a pair of spacer 'ribs detachably disposed transversely of eachplate, the ribs of each plate being adapted to have face to face contactwith the ribs of the other plate, registering mold receiving recesses inthe plates between adjacent ribs, companion mold elements mounted inopposed relationship in the said recesses, mating article cavities andadjacent mating overow cavities in the adiacent faces of the moldelements, and registering wall portions on said companion mold elementsbetween the article and overflow cavities, the outer surface of saidwall portion of each mold element being positioned coplanar with theouter faces of the spacer ribs carried by the plate in which the moldelement is mounted.

2. A device for molding plastic articles comprising companion masterplates, a pair of spacer ribs detachably disposed transversely of eachplate, the ribs of each plate being adapted to have face to face contactwith the ribs of the other plate, registering mold receiving recesses inthel plates between adjacent ribs, companion mold elements mounted inopposed relationship in the said recesses, mating article cavities andadjacent mating overflow cavities in the adjacent faces of the moldelements, registering wall portions on said companion mold elementsbetween the article and overflow cavities, the outer surface of saidwall portion of each mold element being adapted to be positionedcoplanar with the outer faces of the spacer ribs carried by the plate inwhich the mold element is mounted, and means for adjustably securing therespective mold elements in the respective mold receiving recesseswhereby to so aline the outer surfaces of the respective wall portionswith said spacer ribs.

3. A device for molding plastic articles comprising companion inasterplates, a pair of spacer ribs detachably disposed transversely of eachplate, the nbs of each plate being adapted to have face to face contactwith the ribs of the other plate, registering mold receiving recesses inthe plates between adjacent ribs, companion mold elements mounted inopposed relationship in the said recesses, mating article cavities andadjacent mating overflow cavities in the adjacent faces of the moldelements, and registering wall portions on said companion mold elementsbetween the article and overflow cavities, the outer surface of saidwall portion of each mold element providing a cutting edge andpositioned coplanar with the outer faces of the spacer ribs carried bythe plate in which the mold element is mounted.

4. A device for molding plastic articles comprising companion masterplates, a pair of spacer ribs detachably disposed transversely of eachplate, the ribs of each plate being adapted to have face to face contactwith the ribs of the other plate, registering mold receiving recesses inthe plates between adjacent ribs, companion mold elements mounted inopposed relationship with their inner ends in the said recesses, matingarticle cavities and adjacent mating overflow cavities in the adjacentfaces of the mold elements, registering wall portions on said companionmold elements between the article and overflow cavities, the outersurface of said wall portion of each mold element providing a cuttingedge and being adapted to be positioned coplanar with the outer faces ofthe spacer ribs carried by they plate in which the mold element ismounted, and means for adjustably securing the respective mold elementsin the respective mold receiv' ing recesses whereby to so aline theouter surfaces of the respective wall portions with said spacer ribs,said means including a spacer member interposed between the inner end ofeach mold element and the bottom of the recess and a screw extendingthrough the carrying master plate and threadedly engaging the inner endof the mold element.

5. A device for molding plastic articles comprising companion masterplates, a pair of parallel transverse grooves in the adjacent surfacesof the plates, spacer ribs disposed one in each of the grooves, the ribsof each plate being adapted to have face to face contact with the ribsof the other plate, registering mold receiving recesses in the platesbetween adjacent grooves, companion mold elements mounted in matingrelationship in the said recesses, mating article cavities and adjacentmating overow cavities in the mating faces of the mold elements, andregistering wall portions on the mold elements between the article andoverow cavities, the outer surface of said wall portion of each moldelement being positioned coplanar with the outer faces of the spacerribs carried by the plate in which the mold element is mounted.

VOLNEY S. ANDERSON.

